Bridges improved tests and nits (#5128)
This PR adds `exporter_is_compatible_with_pallet_xcm_bridge_hub_router`,
which ensures that our `pallet_xcm_bridge_hub` and
`pallet_xcm_bridge_hub_router` are compatible when handling
`ExportMessage`. Other changes are just small nits and cosmetics which
makes others stuff easier.
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Co-authored-by: Svyatoslav Nikolsky <[email protected]>